Abstract:

The present invention provides solution to the problem involved in
preparation of metal nanosponges using templates and polymers. The
instant invention is successful in providing a simple, template free
single step process for the preparation of metal nanosponges having
porous low density and high surface area. These metal nanosponges were
found to be good self-supported substrates for surface-enhanced Raman
spectroscopy (SERS) and have shown significant anti-bacterial activity.

Claims:

1) A template free and polymer free metal nanosponge.

2) The nanosponge as claimed in claim 1, wherein said metal is selected
from a group comprising gold, silver, platinum, palladium, and copper.

3) The nanosponge as claimed in claim 1, wherein said metal nanosponge is
porous, stable, black in colour, has low density and high surface area.

4) The nanosponge as claimed in claim 3, wherein the porosity is ranging
from about 50 nm to about 100 nm, density is ranging from about 0.5
gcm-3 to about 1 gcm-3 and stable at temperature ranging from
about 25.degree. C. to about 300.degree. C.

5) The nanosponge as claimed in claim 3, wherein the surface area of
silver nanosponge is ranging from about 13 m2/g to about 18
m2/g, preferably about 16 m2/g, gold nanosponge is ranging from
about 41 m2/g to about 45 m2/g, preferably about 43 m2/g,
platinum nanosponge is ranging from about 40 m2/g to about 46
m2/g, preferably about 44 m2/g palladium nanosponge is ranging
from about 78 m2/g to about 84 m2/g, preferably about 81
m2/g and copper nanosponges is ranging from about 48 m2/g to
about 53 m2/g, preferably about 50 m2/g.

6) A process for preparation of template free and polymer free metal
nanosponge, said process comprising steps of:a) mixing equimolar
concentration of one part of metal precursor and five parts of reducing
agent solution to obtain a spongy solid; andb) filtering and washing the
spongy solid followed by drying to obtain the metal nanosponge.

7) The process as claimed in claim 6, wherein said metal precursor is
selected from a group comprising silver nitrate, chloroauric acid,
dihydrogen hexachloroplatinate, palladium dichloride and cuprous nitrate.

8) The process as claimed in claim 6, wherein said equimolar concentration
is about 0.1 M.

9) The process as claimed in claim 6, wherein said metal precursor and
reducing agent are mixed at a volume ratio of about 1:5.

10) The process as claimed in claim 6, wherein said reducing agent is
sodium borohydride.

11) The process as claimed in claim 6, wherein said mixing of metal
precursor solution with reducing agent results in spontaneous formation
of effervescence and nano sized ligament metallic networks which
aggregate to form a black spongy solid floating on the reaction medium.

12) The process as claimed in claim 11, wherein said processing step of
obtaining a spongy solid floating on reaction medium is completed within
a time period of about 5 minutes.

13) Use of template free and polymer free metal nanosponge as substrates
for surface-enhanced Raman Spectroscopy and for anti-bacterial activity.

Description:

FIELD OF THE INVENTION

[0001]The present invention is in relation to the field of nanotechnology.
More particularly, the present invention provides template free metal
nanosponge and also a simple process for the preparation of such metal
nanosponge.

BACKGROUND AND PRIOR ART OF THE INVENTION

[0002]Metal sponges are identified as a new class of materials for their
unique properties such as low density, gas permeability and thermal
conductivity and have the potential to play a major role in adsorption,
catalysis, fuel cells, membranes and sensors. Though significant progress
has been made in making and manipulating high surface area metal oxide
sponges, the same is not true for their metallic counterparts. The most
versatile template based approach, used for the synthesis of porous metal
oxides did not give the desired results with the metals and in
particular, the noble metals such as Ag, Au, Pt and Pd which are
industrially more valuable. For example, in an elegant approach, Mann and
co-workers, synthesized metallic foams of silver and gold using the
polysaccharide, dextran, as the sacrificial template [1]. However, the
macroporous silver foam obtained has the surface area of less than 1
m2/g. More recently, Rao et al [2] have reported the synthesis of
macroporous silver foam with the surface area around 1 m2/g by
calcining the silver salt-surfactant, tritonX-100 composite at
550° C. Cellulose fibers [3] and, poly(ethyleneimine) hydrogel [4]
have also been used as soft templates to prepare porous silver
frameworks. Even, biologically formed porous skeleton was used as a
template to obtain macroporous gold framework [5]. In all these cases,
the template removal needs high temperature calcinations which sinter the
metallic structure and thereby reduces the surface area drastically. The
low temperature route, on the other hand uses colloidal crystals
templates such as silica or latex spheres [6] which involves multi-step
process in addition to the dissolution of templates in organic solvents
or HF. Pattern-forming instabilities during selective dissolution of
silver from Ag--Au alloys reported to give nanoporous gold with
controlled multi-modal pore size distribution [7]. Herein, we report an
instantaneous formation of high surface area noble metal sponges through
a template free, one-step, inexpensive, method. By optimizing a very well
known Oswald ripening process we were able to generate a three
dimensional porous structure made up of nanowire networks. Since this
process involves a simple, room temperature reduction of metal salts with
sodium borohydride, it can be scalable to any amount.

OBJECTIVES OF THE PRESENT INVENTION

[0003]The main objective of the present invention is to provide metal
nanosponges/nano structures.

[0004]Another objective of the present invention is to develop a template
free, single step process for the preparation of metal nanosponges.

[0005]Yet another objective of the present invention is to provide metal
nanosponges which are having high surface area, low density and porous
metal nanosponges.

[0006]Still another objective of the present invention is to provide
template free and polymer free metal nanosponges which can be used in
surface enhanced Raman Spectroscopy [SERS] and also for their
anti-bacterial activity.

STATEMENT OF THE INVENTION

[0007]Accordingly, the present invention provides a template free and
polymer free metal nanosponges; a process for preparation of template
free metal nanosponge, said process comprising steps of: mixing equimolar
concentration of one part of metal precursor and five parts of reducing
agent solution to obtain a spongy solid; and filtering and washing the
spongy solid followed by drying to obtain the metal nanosponge; and use
of template free and polymer free metal nanosponge as substrates for
surface-enhanced Raman Spectroscopy and for anti-bacterial activity.

[0051]The present invention is in relation to a template free and polymer
free metal nanosponge.

[0052]In another embodiment of the present invention said metal is
selected from a group comprising gold, silver, platinum, palladium, and
copper.

[0053]In yet another embodiment of the present invention said metal
nanosponge is porous, stable, black in colour, has low density and high
surface area.

[0054]In still another embodiment of the present invention porosity is
ranging from about 50 nm to about 100 nm, density is ranging from about
0.5 gcm-3 to about 1 gcm-3 and stable at temperature ranging
from about 25° C. to about 300° C.

[0055]In still another embodiment of the present invention the surface
area of silver nanosponge is ranging from about 13 m2/g to about 18
m2/g, preferably about 16 m2/g, gold nanosponge is ranging from
about 41 m2/g to about 45 m2/g, preferably about 43 m2/g,
platinum nanosponge is ranging from about 40 m2/g to about 46
m2/g, preferably about 44 m2/g palladium nanosponge is ranging
from about 78 m2/g to about 84 m2/g, preferably about 81
m2/g and copper nanosponges is ranging from about 48 m2/g to
about 53 m2/g, preferably about 50 m2/g.

[0056]The present invention is in relation to a process for preparation of
template free and polymer free metal nanosponge, said process comprising
steps of: mixing equimolar concentration of one part of metal precursor
and five parts of reducing agent solution to obtain a spongy solid; and
filtering and washing the spongy solid followed by drying to obtain the
metal nanosponge.

[0057]In another embodiment of the present invention said metal precursor
is selected from a group comprising silver nitrate, chloroauric acid,
dihydrogen hexachloroplatinate, palladium dichloride and cuprous nitrate.

[0058]In another embodiment of the present invention said equimolar
concentration is about 0.1 M.

[0059]In yet another embodiment of the present invention said metal
precursor and reducing agent are mixed at a volume ratio of about 1:5.

[0060]In still another embodiment of the present invention said reducing
agent is sodium borohydride.

[0061]In still another embodiment of the present invention said mixing of
metal precursor solution with reducing agent results in spontaneous
formation of effervescence and nano sized ligament metallic networks
which aggregate to form a black spongy solid floating on the reaction
medium.

[0062]In still another embodiment of the present invention said processing
step of obtaining a spongy solid floating on reaction medium is completed
within a time period of about 5 minutes.

[0063]The present invention is in relation to use of template free and
polymer free metal nanosponge as substrates for surface-enhanced Raman
Spectroscopy and for anti-bacterial activity.

[0064]The technology of the instant Application is further elaborated with
the help of following examples. However, the examples should not be
construed to limit the scope of the invention.

Example: 1

Experimental Procedure

[0065]Porous silver sponge has been synthesized by adding 10 ml aqueous
solution of 0.1 M AgNO3 to 50 ml aqueous solution of 0.1 M
NaBH4 (NaBH4/AgNO3 solution volume ratio=5). Addition of
silver nitrate to the borohydride solution resulted in the spontaneous
formation of effervescence (due to the release of hydrogen) with a black
spongy solid floating on the reaction medium. The floating solid was
filtered and washed with distilled water and later dried at room
temperature. The whole reaction can be completed within 5 minutes. To
verify the optimum amount of NaBH4 required, experiments were
carried out at different volume ratios (1, 2, 3 and 4) of
NaBH4/AgNO3 of 0.1 M concentrations. Similarly, the same
synthesis procedure is followed at different concentrations of AgNO3
(1 mM and 2 M respectively) keeping the NaBH4 concentration
constant, 0.1 M. FIG. 1 provides for the schematic representation for the
formation of silver metal nanosponges.

[0066]In a similar method gold nanosponge was synthesized by adding 10 ml
of 0.1 M HAuCl4 to 50 ml of 0.1 M NaBH4 Platinum and palladium
nanosponges were synthesized by adding 10 ml of 0.1 M metal precursors
(H2PtCl6 for platinum and PdCl2 for palladium) to 50 ml of
0.1 M NaBH4. It is also possible to form porous sponges of these
noble metals with different concentrations. Cu/Cu2O nanosponge was
prepared by the addition of 10 ml of 0.1 M copper nitrate solution to 50
ml of 0.1 M NaBH4 solution.

Discussion:

[0067]Porous silver sponge with high surface area can be readily formed
merely by mixing a solution of silver nitrate with borohydride of optimum
concentration. If the concentration of silver nitrate is low, around 1.0
mM, porous silver network does not form, no matter how much amount of 0.1
M sodium borohydride is added. If the concentration of silver nitrate is
0.1 M, addition of equal volume of sodium bororohydride (of 0.1 M
concentration) resulted in a micron sized ligment silver networks.
However, increasing the concentration of borohydride (to 0.2 M) or double
the volume of 0.1 M sodium borohydride gives a very porous network made
up of nanosized ligaments (30 to 50 nm). The formation of silver
nanosponge is favourable when the concentration of silver nitrate and
sodium borohydride solution are kept 0.1 M and above.

[0068]The silver nanosponge prepared with a volume ratio of 1:5 (for 0.1 M
AgNO3 solution: 0.1 M NaBH4 solution) has a surface area of 16
m2/g which is the highest surface area for a silver sponge (prepared
with out any template) reported so far. It is clear from our studies that
to form the metal nanosponge, we need to have some critical amount of
metal ions in solution. If the concentration of metal ions is below the
critical level, it favours the formation of colloidal nanoparticles
stabilized in solution. For example, 1.0 mM colourless silver nitrate
solution gives yellow to dark green colour solution on reduction with
sodium borohydride (1 mM or 0.1 M concentration) due to the dispersion of
silver nanoparticles stabilized by the excess borohydride anions on its
surface. The table 1 below provides list of metal nanosponges and their
surface area. Also, table 2 provides comparison of metal nanosponges
prepared using 0.1M and 2 M solutions of metal precursors and reducing
agent.

[0069]Addition of sodium borohydride to the silver nitrate solution
creates lots of silver nuclei (clusters) which act as the nucleation
centers for further growth. The number of the nucleation sites (reduced
silver sites) formed is directly proportional to the amount of
borohydride added. With time, Oswald ripening occurs fusing the small
nanoparticles to form chained interconnected networks of silver (if the
concentration of silver nitrate is around 0.1 M and above). These
networks aggregate to form a black spongy solid that floats in the
solution. The size of the ligaments in the nanosponge can be tuned by
changing the concentration of sodium borohydride. The FESEM images of
various metal nanosponges are provided in FIGS. 2 to 11.

Example: 2

Stability Studies

[0070]To study the stability of the silver sponge at higher temperatures,
we have heated the as formed sponge at different temperatures and
measured the surface areas of those samples. The sample treated at
200° C. has a surface area of 13 m2/g and sample treated at
300° C. has a surface area of 11 m2/g and a sample treated at
500° C. has a surface area of 1 m2/g. As the temperature
increases, the surface area of the silver sponge decreases. This can be
attributed to the fact that as the temperature increases, nanoparticles
sinter to form bigger particles which further decreases the surface area.
The experimental results obtained in the study of nitrogen
adsorption/desorption isotherms of various metal nanosponges are provided
in FIGS. 12 to 21. Similarly, the X-ray diffraction studies for various
metal nanosponges are provided in FIGS. 22 to 26.

[0071]This porous silver sponge can also be pressed in the form of a
pellet to obtain a monolith without altering much of its surface area.
Pellets were made by applying two different pressures, 1 kN and 10 kN and
their surface areas were also measured. A pellet made of 1 kN pressure
has a surface area of 12 m2/g and for a pellet made of 10 kN, has a
surface area of 9 m2/g. Pellets can be formed of different sizes and
shapes by applying various pressures. The surface area slightly decreases
as the applied pressure increases. The decrease in surface area here is
due to the reduction of void size as well as the fusion of smaller silver
nano ligaments into larger ones. A photograph showing pellets of silver
sponge pressed at 10 kN and 1 kN respectively and cross sectional view of
a silver sponge pellet pressed at 1 kN are showed in FIG. 27.

[0072]The similar procedure applied to obtain porous sponges of other
noble metals like gold, platinum and palladium too. Each of these metal
sponges prepared were having a high surface area for the unsupported
metals reported so far. In all these synthesis procedures, the
concentration of the metal precursor and sodium borohydride was
maintained at 0.1 M and also the volume ratio of the metal salt and the
borohydride solution has been maintained at 1:5 throughout. Irrespective
of the metal present, all the metal sponges obtained were black in color
with a very low density. The respective surface areas for these metal
sponges are, porous gold is 35 m2/g, porous platinum is 44 m2/g
and porous palladium is 81 m2/g. The procedure followed here in to
obtain porous metal sponges is the simplest procedure ever reported and
also an inexpensive, single step room temperature synthesis which can be
scalable to desired amount.

Example: 3

Applications of Metal Nanosponges

[0073]These metal nanosponges were tested for possible applications. The
silver and gold nanosponges were found to be good self-supported
substrates for surface-enhanced Raman spectroscopy (SERS) and also the
silver nanosponge incorporated Whatman filter membrane has shown
significant anti-bacterial activity.

Surface-Enhanced Raman Spectroscopy (SERS)

[0074]The as prepared nanosponges of silver and gold nanosponges were
tested for SERS activity. For this purpose, 20 μl of Rhodamine 6G
(both 10-4 M and 10-6 M) was drop casted onto a glass slide
containing 10 mg of the nanosponge sample (in the form of powder or as a
pellet). Raman spectra were recorded at room temperature using 632 nm
HeNe laser as a source. The characteristic signals for Rhodamine 6G was
enhanced multifold when observed over the Ag and Au substrates whereas
the Rhodamine 6G dye of 10-4 M concentration over the glass slide
without the nanosponge could not be detected (see FIGS. 28 and 29).

Anti-Bacterial Studies

[0075]To study the anti-bacterial activity of the silver, a silver
nanosponge--Whatman composite membrane was prepared by dipping a Whatman
filter paper (125 mm Ashless circles obtained from Whatman Schleicher &
Schuell) in 10 ml of 0.1 M AgNO3 solution for 30 minutes and
followed by dipping it in a 50 ml 0.1 M NaBH4 solution. Immediate
reaction resulted in a dark grey colored membrane. The membrane was
washed several times with Millipore water and dried at room temperature
prior to the study of anti-bacterial activity.

[0076]Anti-bacterial study was done using E. Coli (DH5α). The
bacteria were inoculated in LB (Luria Bertani) broth and grown overnight
at 37° C. in a shaker incubator. The bacterial cells were spread
plated on an agar medium (1.5% agar plates were made for the purpose).
The composite membrane were placed on these plates and incubated
overnight at 37° C. The bacterial growth was observed over the
entire plates except for the zone where the composite membranes were
placed. An inhibition zone was clearly seen surrounding the region of the
membranes (see FIGS. 30 and 31).